Nand string with a redundant memory cell

ABSTRACT

The invention provides methods and apparatus. A NAND memory block has a source select line for selectively coupling one or more strings of series-coupled non-volatile memory cells to a source line, a drain select line for selectively coupling one or more strings of series-coupled non-volatile memory cells to one or more associated bit lines, a plurality of primary rows of memory cells interposed between the source select line and the drain select line and forming a portion of the one or more strings of series-coupled non-volatile memory cells, and one or more redundant rows of memory cells interposed between the source select line and the drain select line and forming a remaining portion of the one or more strings of series-coupled non-volatile memory cells.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 11/394,488, titled “NAND STRING WITH A REDUNDANT MEMORY CELL,” filed Mar. 30, 2006 (allowed), and commonly assigned and incorporated herein by reference.

FIELD

The present disclosure relates generally to memory devices and in particular the present invention relates to a NAND string with a redundant memory cell.

BACKGROUND

Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.

Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the cells, through programming of charge storage or trapping layers or other physical phenomena, determine the data value of each cell. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, cellular telephones and removable memory modules.

A NAND flash memory device is a common type of flash memory device, so called for the logical form in which the basic memory cell configuration is arranged. Typically, the array of memory cells for NAND flash memory devices is arranged such that the control gate of each memory cell of a row of the array is connected to a word line. Columns of the array include strings (often termed NAND strings) of memory cells connected together in series, source to drain, between a pair of select lines, a source select line and a drain select line. The source select line includes a source select gate at each intersection between a NAND string and the source select line, and the drain select line includes a drain select gate at each intersection between a NAND string and the drain select line. The select gates are typically field-effect transistors. Each source select gate is connected to a source line, while each drain select gate is connected to a column bit line.

The memory array is accessed by a row decoder activating a row of memory cells by selecting the word line connected to a control gate of a memory cell. In addition, the word lines connected to the control gates of unselected memory cells of each string are driven to operate the unselected memory cells of each string as pass transistors, so that they pass current in a manner that is unrestricted by their stored data values. Current then flows from the column bit line to the source line through each NAND string via the corresponding select gates, restricted only by the selected memory cells of each string. This places the current-encoded data values of the row of selected memory cells on the column bit lines.

In the event of a defective row, the whole string rows may become defective in that the rows of the string are connected in series. Therefore, in the event of a defective row, the entire memory block containing the defective row is typically replaced with a redundant memory block. However, it is not always practical to have multiple redundant memory blocks, especially in view of the tendency toward decreasing memory device sizes and thus memory array sizes. Moreover, as device sizes decrease, the spacing between adjacent rows (or word lines) of memory cells decreases and the spacing between adjacent bit lines of the memory array decreases, increasing the likelihood of row-to-row and bit-line-to-bit-line shorts.

For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternatives to existing NAND memory devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustration of a NAND memory device, according to an embodiment.

FIG. 2 is a schematic of a NAND memory array in accordance with another embodiment.

FIG. 3 is an illustration of an exemplary memory module, according to another embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments. In the drawings, like numerals describe substantially similar components throughout the several views. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present disclosure is defined only by the appended claims and equivalents thereof.

FIG. 1 is a block diagram illustration of a NAND memory device 102, according to an embodiment of the invention. The memory device 102 may be fabricated as semiconductor device on a semiconductor substrate. For one embodiment, memory device 102 includes an array of flash memory cells 104, an address decoder 106, row access circuitry 108, column access circuitry 110, control circuitry 112, Input/Output (I/O) circuitry 114, and an address buffer 116. Control circuitry 112 is adapted to perform operations of the invention.

Memory device also includes a redundancy circuit 118. In operation, column addresses are sent to column access circuitry 110 and to redundancy circuit 118, and row addresses are sent to row access circuitry 108 and to redundancy circuit 118. Redundancy circuit 118 respectively compares the column addresses and row addresses to addresses of defective primary columns and defective primary rows. When redundancy circuit 118 finds a match, it disables the defective row or column and replaces it with a corresponding redundant row or column.

Memory device 102 may be coupled to an external microprocessor 120, or memory controller, for memory accessing as part of an electronic system. The memory device 102 receives control signals from the processor 120 over a control link 122. The memory cells are used to store data that are accessed via a data (DQ) link 124. Address signals are received via an address link 126 that are decoded at address decoder 106 to access the memory array 104. Address buffer circuit 116 latches the address signals. The memory cells are accessed in response to the control signals and the address signals. It will be appreciated by those skilled in the art that additional circuitry and control signals can be provided, and that the memory device of FIG. 1 has been simplified to help focus on the invention.

The memory array 104 includes memory cells arranged in row and column fashion. For one embodiment, each of the memory cells includes a floating-gate field-effect transistor capable of holding a charge. The cells may be grouped into blocks. Each of the cells within a block can be electrically programmed on an individual basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. Columns of the array include strings of memory cells connected together in series, source to drain, between a pair of select lines. A plurality of primary rows of memory cells form a portion of the strings of memory cells, and one or more redundant rows of memory cells form a another portion of the strings of memory cells in accordance with embodiments of the invention. For one embodiment, the redundant rows are coupled to redundant circuitry 118.

FIG. 2 is a schematic of a NAND memory array 200 as a portion of memory array 104 in accordance with another embodiment of the invention. As shown in FIG. 2, the memory array 200 includes word lines 202 ₁ to 202 _(N) and intersecting bit lines 204 ₁ to 204 _(M). Memory array 200 includes NAND strings 206 ₁ to 206 _(M). Each NAND string includes floating-gate transistors 208 ₁ to 208 _(N), each located at an intersection of a word line 202 and a bit line 204. The floating-gate transistors 208 represent non-volatile memory cells for storage of data. The floating-gate transistors 208 of each NAND string 206 are connected in series source to drain between a source select line 214 and a drain select line 215. Source select line 214 includes a source select gate 210, e.g., a field-effect transistor (FET), at each intersection between a NAND string 206 and source select line 214, and drain select line 215 includes a drain select gate 212, e.g., a field-effect transistor (FET), at each intersection between a NAND string 206 and drain select line 215. In this way, the floating-gate transistors 208 of each NAND string 206 are connected between a source select gate 210 and a drain select gate 212.

A source of each source select gate 210 is connected to a common source line 216. The drain of each source select gate 210 is connected to the source of the first floating-gate transistor 208 of the corresponding NAND string 206. For example, the drain of source select gate 210 ₁ is connected to the source of floating-gate transistor 208 ₁ of the corresponding NAND string 206 ₁. Each source select gate 210 includes a control gate 220.

The drain of each drain select gate 212 is connected to the bit line 204 for the corresponding NAND string at a drain contact 228. For example, the drain of drain select gate 212 ₁ is connected to the bit line 204 ₁ for the corresponding NAND string 206 ₁ at drain contact 228 ₁. The source of each drain select gate 212 is connected to the drain of the last floating-gate transistor 208 _(N) of the corresponding NAND string 206. For example, the source of drain select gate 212 ₁ is connected to the drain of floating-gate transistor 208 _(N) of the corresponding NAND string 206 ₁.

Typical construction of floating-gate transistors 208 includes a source 230 and a drain 232, a floating gate 234, and a control gate 236, as shown in FIG. 2. Floating-gate transistors 208 have their control gates 236 coupled to a word line 202. A column of memory array 200 includes a NAND string 206 and the source and drain select gates connected thereto. A row of the floating-gate transistors 208 are those transistors commonly coupled to a given word line 202.

For one embodiment, one or more of the rows corresponding to the word lines 202, e.g., corresponding to word line 202 ₁ and/or word line 202 _(N) of NAND strings 206, are redundant rows, while the remaining rows, e.g., corresponding to word lines 202 ₂ to word line 202 _(N-1), are primary rows. For another embodiment, memory array 200 corresponds to a memory block of memory array 104 of FIG. 1, and includes one or more redundant rows in addition to the number of rows normally addressable in a memory block, such as 32 primary rows by 16,384 columns. A redundant row or memory cell may be defined as a row or memory cell that is not addressable unless a row of memory cells containing a primary memory cell is identified as defective.

Although the redundant row corresponding to word line 202 ₁ is located adjacent the row of source select gates 210 and the memory cells 208 thereof are respectively coupled directly to the drains of source select gates 210 and the redundant row corresponding to word line 202 _(N) is located adjacent the row of drain select gates 212 and the memory cells 208 thereof are respectively directly coupled to the sources of drain select gates 212, the redundant rows may be located between a pair of primary rows of memory cells with the memory cells 208 thereof respectively coupled in series with the memory cells of the pair of primary rows of memory cells for other embodiments. However, in the event of no defective primary rows, locating a redundant row of memory cells between a row of select gates and a primary row of memory cells acts to reduce the coupling effect of the select gates on the operation of a primary row of memory cells that would otherwise occur if the memory cells of that primary row of memory cells were respectively located directly adjacent the select gates.

In one embodiment, addresses of defective primary rows of memory cells, e.g., addresses of two or more primary rows of memory cells shorted together, are programmed in registers of redundancy circuit 118 of FIG. 1. Redundancy circuit 118 receives addresses of primary rows of memory cells from address decoder 106 of FIG. 1 and compares those addresses to the defective primary row addresses stored therein. For one embodiment, if an address of at least one of the primary rows of memory cells from address decoder 106 matches an address of one of the addresses of two or more primary rows shorted together, redundancy circuit 118 respectively drives the associated redundant row. That is, the primary row whose address matches a defective primary row address is disabled so that it is not responsive to the decoded address, and a match signal activates a corresponding redundant row in place of the primary row. For example, the address of the at least one of the primary rows may be the lowest address of the two or more primary rows shorted together and may have an indicator associated with it that indicates that the succeeding one or more primary rows with the next lowest addresses are shorted to the primary row with the lowest address of the two or more primary rows shorted together. Therefore, redundancy circuit 118 disables the primary row with the lowest address and activates a corresponding one of the redundant rows in place of the primary row with the lowest address. Redundancy circuit 118 also disables the one or more primary rows with the next lowest addresses and respectively activates corresponding redundant rows in place of the one or more primary rows with the next lowest addresses for one embodiment.

For another embodiment, if an address of at least one of the primary rows of memory cells from address decoder 106 matches an address of one of the addresses of two or more primary rows shorted together, the two or more primary rows shorted together are treated as a single row corresponding to one of the addresses of the two or more primary rows shorted together, e.g., the address of the at least one of the primary rows of memory cells from address decoder 106 that matches the address of one of the addresses of two or more primary rows shorted together. Redundancy circuit 118 then respectively replaces the remaining one or more primary rows shorted together with one or more corresponding redundant rows.

One problem related to NAND memory devices is the rows having memory cells located at the ends of the NAND strings and respectively connected directly to the source select gate and the drain select gate have different programming characteristics than those rows that are situated away from the ends. This is due to coupling effects between the end rows and their respective select lines.

For another embodiment, if none of the addresses of the primary rows of memory cells from address decoder 106 matches an address of a defective row, redundancy circuit 118 drives the memory cells, e.g., corresponding to word line 202 ₁ of FIG. 2, located at one end of the NAND strings as unselected rows. Redundancy circuit 118 further drives the memory cells, e.g., corresponding to word line 202 _(N), located at the other end of the NAND strings as unselected rows. Driving the rows respectively disposed at either end of the NAND strings as unselected rows acts to substantially reduce coupling effects between the end rows of memory cells and their respective select lines.

FIG. 3 is an illustration of an exemplary memory module 300. Memory module 300 is illustrated as a memory card, although the concepts discussed with reference to memory module 300 are applicable to other types of removable or portable memory, e.g., USB flash drives, and are intended to be within the scope of “memory module” as used herein. In addition, although one example form factor is depicted in FIG. 3, these concepts are applicable to other form factors as well.

In some embodiments, memory module 300 will include a housing 305 (as depicted) to enclose one or more memory devices 310, though such a housing is not essential to all devices or device applications. At least one memory device 310 may be a NAND flash memory device, such as NAND memory device 102 of FIG. 1, having a memory array, such as memory array 200 of FIG. 2, in accordance with the invention. Where present, the housing 305 includes one or more contacts 315 for communication with a host device. Examples of host devices include digital cameras, digital recording and playback devices, PDAs, personal computers, memory card readers, interface hubs and the like. For some embodiments, the contacts 315 are in the form of a standardized interface. For example, with a USB flash drive, the contacts 315 might be in the form of a USB Type-A male connector. For some embodiments, the contacts 315 are in the form of a semi-proprietary interface, such as might be found on CompactFlash™ memory cards licensed by SanDisk Corporation, Memory Stick™ memory cards licensed by Sony Corporation, SD Secure Digital™ memory cards licensed by Toshiba Corporation and the like. In general, however, contacts 315 provide an interface for passing control, address and/or data signals between the memory module 300 and a host having compatible receptors for the contacts 315.

The memory module 300 may optionally include additional circuitry 320 which may be one or more integrated circuits and/or discrete components. For some embodiments, the additional circuitry 320 may include a memory controller for controlling access across multiple memory devices 310 and/or for providing a translation layer between an external host and a memory device 310. For example, there may not be a one-to-one correspondence between the number of contacts 315 and a number of I/O connections to the one or more memory devices 310. Thus, a memory controller could selectively couple an I/O connection (not shown in FIG. 3) of a memory device 310 to receive the appropriate signal at the appropriate I/O connection at the appropriate time or to provide the appropriate signal at the appropriate contact 315 at the appropriate time. Similarly, the communication protocol between a host and the memory module 300 may be different than what is required for access of a memory device 310. A memory controller could then translate the command sequences received from a host into the appropriate command sequences to achieve the desired access to the memory device 310. Such translation may further include changes in signal voltage levels in addition to command sequences.

The additional circuitry 320 may further include functionality unrelated to control of a memory device 310 such as logic functions as might be performed by an ASIC (application specific integrated circuit). Also, the additional circuitry 320 may include circuitry to restrict read or write access to the memory module 300, such as password protection, biometrics or the like. The additional circuitry 320 may include circuitry to indicate a status of the memory module 300. For example, the additional circuitry 320 may include functionality to determine whether power is being supplied to the memory module 300 and whether the memory module 300 is currently being accessed, and to display an indication of its status, such as a solid light while powered and a flashing light while being accessed. The additional circuitry 320 may further include passive devices, such as decoupling capacitors to help regulate power requirements within the memory module 300.

CONCLUSION

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof. 

1. A NAND memory block, comprising: a plurality of strings of series-coupled memory cells; wherein each of the plurality of strings of series-coupled memory cells contains a plurality of primary memory cells coupled in series; wherein at least one of the strings of series-coupled memory cells contains two or more redundant memory cells coupled in series with the plurality of primary memory cells; and wherein a first redundant memory cell is formed at one end of the at least one of the strings of series-coupled memory cells and a second redundant memory cell is formed at an opposite end of the at least one of the strings of series-coupled memory cells.
 2. The NAND memory block of claim 1, wherein the first redundant memory cell is coupled to a source select gate of the at least one of the strings of series-coupled memory cells.
 3. The NAND memory block of claim 2, wherein the source select gate of the at least one of the strings of series-coupled memory cells selectively couples the at least one of the strings of series-coupled memory cells to a source line.
 4. The NAND memory block of claim 3, wherein the second redundant memory cell is coupled to a drain select gate of the at least one of the strings of series-coupled memory cells.
 5. The NAND memory block of claim 4, wherein the drain select gate of the at least one of the strings of series-coupled memory cells selectively couples the at least one of the strings of series-coupled memory cells to a bit line.
 6. A NAND memory block, comprising: a plurality of strings of series-coupled memory cells; wherein each of the plurality of strings of series-coupled memory cells contains a plurality of primary memory cells coupled in series; wherein at least one of the strings of series-coupled memory cells contains one or more redundant memory cells coupled in series with the plurality of primary memory cells; and wherein the one or more redundant memory cells of the at least one of the strings of series-coupled non-volatile memory cells are addressable only if a primary memory cell of the at least one of the strings of series-coupled memory cells is identified as defective.
 7. A NAND memory block, comprising: a plurality of strings of series-coupled memory cells; wherein each of the plurality of strings of series-coupled memory cells contains a number of primary memory cells coupled in series; wherein at least one of the strings of series-coupled memory cells contains one or more redundant memory cells coupled in series with the number of primary memory cells; and wherein the number of primary memory cells is equal to a number of addressable rows of the memory block.
 8. The NAND memory block of claim 7, wherein the addressable rows of the memory block are primary rows of the memory block.
 9. The NAND memory block of claim 8, wherein the one or more redundant memory cells are respectively contained in one or more redundant rows of the memory block.
 10. The NAND memory block of claim 9, wherein the one or more redundant rows of memory cells are coupled to a redundancy circuit of a memory device.
 11. The NAND memory block of claim 10, wherein the redundancy circuit is configured to store addresses of defective primary rows of the memory block.
 12. The NAND memory block of claim 11, wherein the redundancy circuit is configured to compare the addresses of the defective primary rows to addresses of primary rows received from an address decoder of the memory device and to replace a primary row with a redundant row when the address of that primary row matches an address of a defective primary row.
 13. A method of operating a NAND memory device, comprising: replacing one or more of two or more defective primary rows of memory cells with one or more redundant rows of memory cells; and operating the two or more defective primary rows as a single primary row; wherein the two or more defective primary rows of memory cells form a first portion of one or more strings of series-coupled memory cells; wherein the one or more redundant rows of memory cells form a second portion of the one or more strings of series-coupled memory cells; and wherein the second portion of one of the strings of series-coupled memory cells is coupled in series with the first portion of that string of series-coupled non-volatile memory cells.
 14. The method of claim 13, wherein the two or more defective primary rows are two or more primary rows shorted together.
 15. A method of operating a NAND memory device, comprising: when an address of at least one primary row of memory cells matches an address of one row of two or more primary rows of memory cells shorted together, operating the two or more shorted together primary rows of memory cells as a single primary row of memory cells; and replacing one or more remaining rows of the two or more shorted together primary rows of memory cells with one or more redundant rows of memory cells; wherein the two or more shorted together primary rows of memory cells form a first portion of one or more strings of series-coupled memory cells; wherein the one or more redundant rows of memory cells form a second portion of the one or more strings of series-coupled memory cells; and wherein the second portion of one of the strings of series-coupled memory cells is coupled in series with the first portion of that string of series-coupled memory cells.
 16. The method of claim 15, further comprising: receiving the address of the at least one primary row of memory cells at a redundancy circuit of the NAND memory device from an address decoder of the NAND memory device; and comparing, at the redundancy circuit, the address of the at least one primary row of memory cells to addresses, stored in the redundancy circuit, of rows of two or more primary rows of memory cells shorted together.
 17. A NAND memory array, comprising: a plurality of memory blocks, each memory block comprising: a plurality of strings of series-coupled non-volatile memory cells, each string of series-coupled non-volatile memory cells interposed between source and drain select gates; wherein each of the plurality of strings of series-coupled non-volatile memory cells contains a plurality of primary memory cells coupled in series; and wherein at least one of the strings of series-coupled non-volatile memory cells contains two or more redundant memory cells coupled in series with the plurality of primary memory cells, the two or more redundant memory cells comprising a first redundant memory cell formed at one end of the at least one of the strings of series-coupled non-volatile memory cells and a second redundant memory cell formed at an opposite end of the at least one of the strings of series-coupled non-volatile memory cells.
 18. The NAND memory array of claim 17, wherein each source select gate selectively couples a corresponding string of non-volatile memory cells to a source line and wherein each drain select gate selectively couples the corresponding string of non-volatile memory cells to a bit line.
 19. A NAND memory array, comprising: a plurality of memory blocks, each memory block comprising: a plurality of strings of series-coupled non-volatile memory cells, each string of series-coupled non-volatile memory cells interposed between source and drain select gates; wherein each of the plurality of strings of series-coupled non-volatile memory cells contains a plurality of primary memory cells coupled in series; and wherein at least one of the strings of series-coupled non-volatile memory cells contains two or more redundant memory cells coupled in series with the plurality of primary memory cells, the two or more redundant memory cells comprising a first redundant memory cell coupled directly to the source select gate of the at least one of the strings of series-coupled non-volatile memory cells and a second redundant memory cell coupled directly to the drain select gate of the at least one of the strings of series-coupled non-volatile memory cells.
 20. The NAND memory array of claim 19, wherein the two or more redundant memory cells of the at least one of the strings of series-coupled non-volatile memory cells are addressable only if a primary memory cell of the at least one of the strings of series-coupled non-volatile memory cell is identified as defective. 